Cross-point memory and methods for fabrication of same

ABSTRACT

The disclosed technology relates generally to integrated circuit devices, and in particular to cross-point memory arrays and methods for fabricating the same. In one aspect, a memory device of the memory array comprises a substrate and a memory cell stack formed between and electrically connected to first and second conductive lines. The memory cell stack comprises a first memory element over the substrate and a second memory element formed over the first element, wherein one of the first and second memory elements comprises a storage element and the other of the first and second memory elements comprises a selector element. The memory cell stack additionally comprises a first pair of sidewalls opposing each other and a second pair of sidewalls opposing each other and intersecting the first pair of sidewalls. The memory device additionally comprises first protective dielectric insulating materials formed on a lower portion of the first pair of sidewalls and an isolation dielectric formed on the first protective dielectric insulating material and further formed on an upper portion of the first pair of sidewalls.

REFERENCE TO RELATED APPLICATIONS

The present application for patent is a continuation of U.S. patentapplication Ser. No. 15/858,811 by Sciarrillo, entitled “Cross-PointMemory and Methods for Fabrication of Same,” filed Dec. 29, 2017, whichis a divisional of U.S. patent application Ser. No. 15/398,475 bySciarrillo, entitled “Cross-Point Memory and Methods for Fabrication ofSame,” filed Jan. 4, 2017, which is a divisional of U.S. patentapplication Ser. No. 14/189,265 by Sciarrillo, entitled “Cross-PointMemory and Methods for Fabrication of Same,” filed Feb. 25, 2014,assigned to the assignee hereof, and each of which is expresslyincorporated by reference in its entirety herein.

This application is related to U.S. patent application Ser. No.14/189,490 by Ravasio et al., entitled “Cross-Point Memory and Methodsfor Fabrication of Same,” filed Feb. 25, 2014 and U.S. patentapplication Ser. No. 14/189,323 by Song et al., entitled “SemiconductorStructures Including Liners Comprising Alucone and Related Methods,”filed Feb. 25, 2014.

BACKGROUND OF THE INVENTION Field of the Invention

Subject matter disclosed herein generally relates to integrated circuitdevices, and in particular to cross-point memory arrays and methods forfabricating the same.

Description of the Related Art

Generally, a cross-point memory array refers to a memory array havingmemory elements disposed and electrically connected at cross junctionsbetween a first set of conductive lines (e.g., word lines) and a secondset of conductive lines (e.g., bit lines) overlapping and crossing thefirst set of conductive lines. Some cross-point memory arraysincorporate memory elements whose resistance can be changed by a signalapplied across the memory elements. Examples of resistance changematerials include phase change materials.

Fabrication of cross-point memory arrays can include patterning thememory elements having multiple elements in a stacked configuration.Patterning the memory elements, which often involve etching at leastportions of a stack of materials to form a stacked structure, forexample a stack of lines, can result in undesirable effects such asdamaging and contaminating sidewalls of the stacked structure beingetched. Thus, there is a need for fabrication methods that can minimizesuch undesirable effects.

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out in the concludingportion of the specification. However, organization and/or method ofoperation, together with certain objects, features, and/or advantagesthereof, may be better understood by reference to the following detaileddescription if read with the accompanying drawings in which:

FIG. 1 is a schematic three-dimensional depiction of a memory cellaccording to some embodiments.

FIGS. 2A-2I are schematic three-dimensional depictions of intermediatestructures of a memory cell at various stages of fabrication accordingto some embodiments.

FIG. 3A-3C are horizontal cross-sectional views of structures of amemory cell according to some embodiments.

Features in the drawings are not necessarily drawn to scale and mayextend in different directions from that illustrated. While various axesand directions are illustrated to facilitate the discussion herein, itwill be appreciated that the features may extend in differentdirections.

DETAILED DESCRIPTION

Generally, a cross-point memory array refers to a memory array havingmemory cells disposed and electrically connected at cross-junctionsbetween a first set of conductive lines (e.g., word lines) and a secondset of conductive lines (e.g., bit lines) overlapping and crossing thefirst set of conductive lines. The memory cells can change their memorystate in response to a signal, which can be an electrical signal such asa voltage or a current pulse. Some memory cells, sometimes called theresistance change cells, incorporate resistance change materials whoseelectrical resistance can be changed by the electrical signals providedthrough the first and second conductive lines connected to theresistance change materials. One category of such resistance changematerials is that of phase change materials.

The memory cells in a cross-point memory array can include multipleelements arranged in a stacked configuration. For example, a memory cellcan include a storage element and a selector element, as well aselectrodes electrically connecting the storage and selector elementswith each other and to conductive access lines. Patterning the memorycell having the multiple elements, which often involves etching at leastportions of a stack of materials to form a stacked structure, forexample a stack of lines, can result in undesirable effects such asdamaging and/or contaminating sidewalls of the stacked structure beingetched. For example, a particularly efficient patterning scheme candeposit blanket layers for the conductive lines and employ twopatterning processes to pattern all three of a lower line, a crossingupper line and a pillar of memory cell layers between them. However,during etching, sidewalls of one of the storage or selector elements canbecome damaged chemically and/or physically, and by-products from suchdamage can become deposited elsewhere, for example on the other of thestorage or selector elements. Such contamination can be thermally“driven-in” in subsequent processes, thereby causing unintended andundesirable effects on the final device characteristics as well asfabrication yield. Thus, there is need for fabrication methods that canminimize such undesirable effects.

A method of fabricating a memory device is disclosed below according tosome embodiments, including forming a memory stack structure which caninclude, for example, storage and selector elements arranged in astacked configuration. The method includes forming protective dielectricmaterials on portions of sidewalls of the memory stack structures by,for example, first forming an initial protective dielectric materialthat covers sidewalls of the entire stack structure and removing aportion thereof to partially expose other portions of sidewalls of thememory stack structures. The protected and exposed portions of thememory stack structure can be disposed vertically adjacent to oneanother. For example, sidewalls of one of the storage or the selectorelements can be exposed while leaving the sidewalls of the remaining oneof the storage or the selector elements covered by the dielectricmaterial. The element whose sidewalls remain covered can be, forexample, the upper one of the storage and selection elements, which canbe the element that is more prone to damage by subsequent processes, orthe element whose elemental composition, when incorporated into otherportions of the memory stack and/or the memory device, can detrimentallyaffect the performance and/or yield of the memory device.

FIG. 1 depicts a memory cell 10 in a cross-point memory array accordingto one embodiment. The memory cell 10 in FIG. 1 is a resistance changememory cell arranged in a stacked configuration between a column line 20extending in a y-direction and a row line 22 extending in anx-direction. While a single memory cell 10 interposed between one columnline 20 and one row line 22 is disclosed, it will be understood that amemory array can include additional memory cells formed at a subset ofcrossings between a plurality of column lines 20 and a plurality of rowlines 22. Column lines 20 can alternately be referred to as bit lines,or more generally as digit lines, and row lines 22 can alternately bereferred to as word lines, or more generally as access lines, byconvention in the industry, although these designations and associateddriver circuitry can also be reversed. The row and column lines 22 and20 are conductive lines configured to carry electrical signals such as,for example, a voltage or a current pulse. The memory cell 10 includes amemory cell stack 30 including a selector element 38 and a storageelement 34, and in the illustrated embodiment these elements areseparated by a middle electrode 36. The memory cell 10 additionallyincludes a first electrode 40 between the selector element 38 and therow line 22 and a second electrode 32 between the column line 20 and thestorage element 34. The electrodes, which can serve as electricalconnectors, can also prevent unwanted material reaction orinterdiffusion and can also serve as heating elements in operation.

Embodiments modified from the illustrated embodiments of FIG. 1 arepossible. For example, while the illustrated embodiment in FIG. 1 showsthe first electrode 40 configured as a line structure laterally confinedin one dimension and extending above the column line 22 and the secondelectrode 32 configured as a structure laterally confined in twodimensions, the opposite configuration is possible, where the firstelectrode 40 is configured as a structure laterally confined in twodimensions and the second electrode 32 is configured as a line structurelaterally confined in one dimension and extending below the row line 20.In other embodiments, both the first and second electrodes 40 and 32 canbe laterally confined in one or two dimensions. In other embodiments,the positions of the storage element 34 and the selector element 38within a stack configuration may be interchanged with one another. Inyet other embodiments, the selector element 38 may be omitted. In yetother embodiments, any one of the first, second, and middle electrodesmay be omitted, depending upon compatibility or reactivity of adjacentmaterials. Additionally, the “row” and “column” designations areinterchangeable, and the rows and columns are illustrated as beingperpendicular to one another but may cross one another at other than90°.

Examples of the storage element 34 include a chalcogenide-based phasechange material-based storage element, a resistive random access memory(RRAM) storage element (e.g., NiO, HfO₂, ZrO₂, Cu₂O, TaO₂, Ta₂O₅, TiO₂,SiO₂, A₁2O₃), a conductive bridge random access memory (CBRAM) storageelement (e.g., metal-doped chalcogenide), and/or a spin transfer torquerandom access memory (STT-RAM) storage element, among other types ofmemory cells.

Examples of the selector element 38 include a two terminal device (e.g.,a switch), such as a diode, an ovonic threshold switch (OTS), a tunneljunction, or a mixed ionic electronic conduction switch (MIEC), amongother two terminal devices. Alternatively, examples of the selectorelement 38 include a three terminal device (e.g., a switch), such as afield effect transistor (FET) or a bipolar junction transistor (BJT),among other switching elements.

In some embodiments, one or both of the storage and selector elements 34and 38 can comprise chalcogenide materials. When both storage andselector elements 34 and 38 comprise chalcogenide materials, the storageelement 34 can comprise a chalcogenide material that can undergo a phasechange that is nonvolatile at room temperature. On the other hand, theselector element 38 can comprise a chalcogenide material that does notundergo a similar nonvolatile phase change.

In some embodiments, the storage element 34 includes a phase changematerial that includes chalcogenide compositions such as an alloyincluding at least two of the elements within theindium(In)-antimony(Sb)-tellurium(Te) (IST) alloy system, e.g.,In₂Sb₂Te₅, IniSb₂Te₄, IniSb₄Te₇, etc., an alloy including at least twoof the elements within the germanium(Ge)-antimony(Sb)-tellurium(Te)(GST) alloy system, e.g., Ge₈Sb₅Te₈, Ge₂Sb₂Te₅, GeiSb₂Te₄, GeiSb₄Te₇,Ge₄Sb₄Te₇, etc., among other chalcogenide alloy systems. The hyphenatedchemical composition notation, as used herein, indicates the elementsincluded in a particular mixture or compound, and is intended torepresent all stoichiometries involving the indicated elements. Otherchalcogenide alloy systems that can be used in phase change storageelements include Ge-Te, In-Se, Sb-Te, Ga-Sb, In-Sb, As-Te, Al-Te,In-Ge-Te, Ge-Sb-Te, Te-Ge-As, In-Sb-Te, Te-Sn-Se, Ge-Se-Ga, Bi-Se-Sb,Ga-Se-Te, Sn-Sb-Te, In-Sb-Ge, Te-Ge-Sb-S, Te-Ge-Sn-O, Te-Ge-Sn-Au,Pd-Te-Ge-Sn, In-Se-Ti-Co, Ge-Sb-Te-Pd, Ge-Sb-Te-Co, Sb-Te-Bi-Se,Ag-In-Sb-Te, Ge-Sb-Se-Te, Ge-Sn-Sb-Te, Ge-Te-Sn-Ni, Ge-Te-Sn-Pd, andGe-Te-Sn-Pt, for example.

In some embodiments, the selector element 38 includes a chalcogenidematerial electrically coupled to the storage element 34 through themiddle electrode 36 on one side and electrically connected to the rowline 22 through the first electrode 40 on the other side. A selectordevice having a chalcogenide material can sometimes be referred to as anOvonic Threshold Switch (OTS). An OTS may include a chalcogenidecomposition including any one of the chalcogenide alloy systemsdescribed above for the storage element 34. In addition, the selectorelement may further comprise an element to suppress crystallization,such as arsenic (As). Examples of OTS materials include Te-As-Ge-Si,Ge-Te-Pb, Ge-Se-Te, Al-As-Te, Se-As-Ge-Si, Se-As-Ge-C, Se-Te-Ge-Si,Ge-Sb-Te-Se, Ge-Bi-Te-Se, Ge-As-Sb-Se, Ge-As-Bi-Te, and Ge-As-Bi-Se,among others.

The electrodes 32, 36 and 40 can comprise materials that electricallyconnect the operational elements of the memory cell but preventreactions among the materials. For example, where the storage element 34and the selector element 38 comprise chalcogenide materials, it may beadvantageous to place non-reactive conductors between these elements toprevent interdiffusion of their materials, and also between theseelements and their respective neighboring materials, particularlyconductive lines comprising metals. Examples of suitable electrodematerials include one or more conductive and semiconductive materialssuitable for forming electrodes including, for example, carbon (C);n-doped polysilicon and p-doped polysilicon; metals including Al, Cu,Ni, Cr, Co, Ru, Rh, Pd, Ag, Pt, Au, Jr, Ta, and W; conductive metalnitrides including TiN, TaN, WN, and TaCN; conductive metal silicidesincluding tantalum silicides, tungsten silicides, nickel silicides,cobalt silicides and titanium silicides; and conductive metal oxidesincluding RuO₂.

In addition, row and column lines 22 and 20 can comprise any conductiveand/or semiconductive material suitable for forming conductive lines tocarry electrical current for accessing the memory cells within thememory array. Examples of conductive/semiconductive materials suitablefor forming row and column lines 22 and 20 include n-doped polysilicon,p-doped polysilicon; metals including Al, Cu, and W; conductive metalnitrides including TiN, TaN, and TaCN; and other conductive materials.In embodiments where one of electrodes 32 or 40 is omitted, the row line22 or the column line 20 can serve as the electrodes in addition tocarrying electrical signals.

In the following, it will be understood that while some embodiments ofmemory cells may be described as having certain features pertaining tostorage elements and selector elements that include chalcogenidematerials, the embodiments are not limited to such storage and selectormaterials. For example, in some embodiments, a memory cell can include astorage element including a chalcogenide phase change material, whileincluding a selector element that does not include a chalcogenidematerial, such as a bipolar junction transistor or a diode. In someother embodiments, another material with similar electrical behavior asthe chalcogenide phase change material may be used in place of thatchalcogenide phase change material. In some other embodiments, a memorycell can include a selector element including an Ovonic thresholdswitching material, while including a storage element that does notinclude a chalcogenide material, such an oxide-based resistance changeoxide material. In yet other embodiments, a memory cell can include achalcogenide phase change material having both storage and selectorfunctionalities without separate selector elements for these functions.

FIGS. 2A-2I are schematic three-dimensional depictions of intermediatememory array structures illustrating a method of fabricating a memorycell stack partially covered with protective dielectric materials,depicted at various stages of fabrication according to some embodiments.

While not shown for clarity and ease of illustration, it will beunderstood that the illustrated array structures are formed over asubstrate, which can include, among other things, various peripheral andsupporting circuitry, for instance CMOS transistors that form a part ofcolumn and row driver circuitry and sense amplifier circuitry, as wellas sockets and wiring that connect such circuitry to the memory arraythrough the columns and rows described above. In addition, the substratemay include one or more memory arrays, or “decks” of arrays. As usedherein, the term substrate includes a bulk semiconductor substrate aswell as integrated structures formed thereover.

As used herein and throughout the specification, “subtractivepatterning” refers to a process sequence where structures to be definedare patterned by the removal of material. For example, a “subtractivepatterning process” may include first lithographically providing etchmask structures overlapping areas to be patterned, followed by etching,such that materials in areas covered by the mask structures areprotected while materials in exposed areas are removed by the etchremoval process. The set of process steps for subtractive-patterning astack of layers, can include, for example, providing an etch maskpattern that can comprise one or more of a resist, hard mask, andanti-reflective coating. The resist maybe be a photoresist patterned bya photolithography process. Other lithographic techniques are alsopossible, including processes without hard mask layers. The etch maskpattern blocks areas covered by the mask pattern to protect theunderlying material from being etched (e.g., wet or dry), while the etchmask pattern exposes areas not covered by the mask pattern to etch theexposed region.

Referring to intermediate array structure 100 a of FIG. 2A, according tosome embodiments, the method of fabricating the memory cell includesforming a lower conductive material 22 a over a substrate (not shown),forming a cell material stack 30 a over the lower conductive material 22a, and forming a first hard mask material 42 a over the cell materialstack 30 a. Forming the cell material stack 30 a in turn includesforming a lower electrode material 40 a on the lower conductive material22 a, forming a selector element material 38 a on the lower electrodematerial 40 a, forming a middle electrode material 36 a on the selectorelement material 38 a, forming a storage element material 34 a on themiddle electrode material 36 a, forming an upper electrode material 32 aon the storage element material 34 a, and forming a first hard maskmaterial 42 a on the upper electrode material 32 a. The foregoingfeatures may be formed, for example, by deposition techniques such asphysical deposition (PVD), chemical vapor deposition (CVD), and atomiclayer deposition (ALD), among other deposition techniques. Each featuremay be initially formed as a blanket layer over an entire substrate,such as a wafer.

Referring to intermediate array structure 100 b of FIG. 2B, according tosome embodiments, the method includes patterning the stack of theintermediate array structure 100 a of FIG. 2A to form a lower line stack44 a extending in the x-direction. Subtractive patterning the lower linestack 44 a includes forming an etch mask line pattern (e.g., photoresistpattern, not shown) comprising lines extending in the x-direction andetching the material stack between exposed regions between the etch maskpattern lines. The resulting lower line stack 44 a is formed by etching,starting from the top, the first hard mask material 42 a, the cellmaterial stack 30 a, and the lower conductive material 22 a of FIG. 2Ato form the lower line stack 44 a including a lower conductive line 22on the substrate, a cell line stack 30 b on the lower conductive line22, and a first hard mask line 42 b on the cell line stack 30 b. Thecell line stack 30 b includes a lower electrode line 40 on the lowerconductive line 22, a selector element line 38 b on the lower electrodeline 40, a middle electrode line 36 b on the selector element line 38 b,a storage element line 34 b on the middle electrode line 36 b, and anupper electrode line 32 b on the storage element line 34 b. It will beappreciated that under some circumstances, the first hard mask material42 a may be substantially removed near the completion of the subtractivepatterning process that forms the lower line stack 44 a. It will beappreciated that only small segments of two lines are shown, but inpractice many parallel lines may be formed, extending across a memoryarray region.

Subtractive patterning as described above involves removing variousmaterials including the hard mask material 42 a and the cell materialstack 30 a from exposed regions of the material stack of FIG. 2A betweenetch mask (e.g., photoresist) lines. As discussed above, the variousmaterials can include, storage and selector materials 34 a and 38 aformed of, for example, chalcogenide materials, and the lower, middle,and upper electrode materials 40 a, 36 a, and 32 a formed of, forexample, carbon or other conductive materials, and the hard maskmaterial formed of, for example, a dielectric material, at least some ofwhich are removed using dry etch removal processes, such as reactive ionetching processes.

Dry etch processes are often tailored for specific material compositionsand a particular physical profile desired. Some dry etch processes mayutilize chemical reactions by employing various etchants comprising, forexample, a halide, such as a fluorine-, chlorine-, bromine- or iodine-containing vapor compound, to form a volatile chemical compound withthe material being removed. Some other dry etch processes may utilizephysical energy by employing acceleration of charged species which maybe etchants themselves, or a combination of the etchants and otherspecies such as inert ions. Yet some other dry etch processes mayutilize a combination of both chemical reactions and physical energy foroptimized etching performance.

It will be appreciated that some dry etch processes for patterning thelower line stack 44 a of FIG. 2B having several different types ofmaterials can sometimes leave some portions of the sidewalls of lowerline stack damaged because of the chemical and the physical processesinvolved in the etch process as discussed above. Under somecircumstances, such damaged sidewalls can result in undesirableelectrical properties of the resulting memory cells, such as a shift inthe threshold/switching voltage of the memory cells, as one example.

In addition, it will be appreciated that some dry etch processes forpatterning the lower line stack 44 a of FIG. 2B having several differenttypes of materials can involve multiple sub-processes, where eachsub-process may be tailored to remove a specific material within thelower line stack. However, a sub-process tailored for removing aparticular material can result in one of several undesirableconsequences. For example, without being bound to any theory, a materialbeing removed can redeposit elsewhere without being fully volatilizedand removed from a process chamber. For example, after removingmaterials up to and including the middle electrode material 36 a, duringa sub-process tailored to remove the selector element material 38 a toform the selector element line 38 b, atoms, molecules, or clusters ofthe selector element material 38 a can be released and re-depositedelsewhere. For example, the released atoms, molecules, or clusters ofthe selector element material 38 a, which can include any of the exampleelements of the chalcogenide material described above (e.g., As and/orSe), can re-deposit on sidewalls that have already been formed, such assidewalls of the storage element line 34 b. Materials released from theselector can contaminate the sidewalls or the bulk material of thestorage element line 34 b, resulting in unintended change in electricalperformance of the resulting memory cells, such as a shift in thethreshold/switching voltage of the memory cells, to name one example.Similarly, material from the lower conductive material 22 a canredeposit on sidewalls and contaminate the selector element materialand/or the storage element material during the etch of the lowerconductive material 22 a.

To avoid such contamination and unintended changes in the electricalperformance of the memory cells that can result from the dry etchprocesses described with respect to FIG. 2B, it can be advantageous toclean at least portions of the sidewalls of the lower line stack 44 aafter partially or fully etching to form the lower line stack 44 a.However, a cleaning process designed for cleaning one portion of thesidewalls itself can also lead to undesirable results, such as untendedremoval of material from the portion of the sidewalls and redepositionat other portions of the sidewalls, and vice versa. By way of anexample, a wet cleaning process may be employed after fully etching toform the lower line stack 44 a to remove contaminants (e.g., Se) fromthe sidewalls of the storage element line 34 b. Such cleaning processcan, however, result in unintended removal of additional contaminants(e.g., additional Se) from the exposed sidewalls of the selector elementline 38 b, resulting in further contamination of the storage elementline 34 b. Thus, in the descriptions that follow, methods to preventsuch cross-contamination are disclosed.

Referring to intermediate array structure 100 c of FIG. 2C, according tosome embodiments, fabricating the memory cell stack additionallyincludes forming an initial protective dielectric material 46 a coveringsidewalls of the lower line stack 44 a of FIG. 2B including sidewalls ofthe lower conductive line 22, sidewalls of any remaining portions of thecell line stack 30 b and sidewalls of the first hard mask line 42 b, toform a lower line stack 44 b having sidewalls covered with the initialprotective dielectric material 46 a.

In some embodiments, forming the initial protective dielectric material46 a includes depositing a conformal protective dielectric material,which uniformly and substantially covers all exposed surfaces of theintermediate structure 100 c of FIG. 2C, including the sidewalls of thelower line stack 44 a and inter-line stack regions between adjacentlower line stacks 44 a. As used herein, a conformal initial protectivedielectric material 46 a has thicknesses on various portions of exposedsurfaces that are substantially the same. Thus, when conformal, theinitial protective dielectric material 46 a has substantially the samethicknesses on top surface and sidewalls of the lower line stack 44 b,as well as on the substrate between adjacent lower line stacks 44 b.

The initial protective dielectric material 46 a can include a suitabledielectric to protect at least portions of the lower line stack 44 bfrom subsequent processes, such as subsequent cleaning processes. Theinitial protective dielectric material 46 a can include oxide or nitridematerials, such as silicon oxide (e.g., SiO2), aluminum oxide (e.g.,A1203) and silicon nitride (e.g., Si3N4), among others. In general, theinitial protective dielectric material 46 a can be formed by a suitableconformal deposition technique such as chemical vapor deposition (CVD)and atomic layer deposition (ALD), including thermal and plasmadeposition techniques.

Referring to intermediate array structure 100 d of FIG. 2D, fabricatingthe memory cell additionally includes, according to some embodiments,removing at least an upper portion of the initial protective dielectricmaterial 46 a of FIG. 2C to expose at least an upper portion of thesidewalls of the lower line stack 44 b, thereby forming a lower linestack 44 c of FIG. 2D having a portion of the initial protectivedielectric material 46 a removed therefrom. The resulting protectivedielectric material 46 b remains, covering at least a lower portion ofthe sidewalls of the lower line stack 44 c.

In some embodiments, removing at least an upper portion of the initialprotective dielectric materials includes a performing a “spacer etchprocess,” which is a directional etch process designed to remove theinitial protective dielectric material 46 a predominantly in a verticaldirection (z-direction) such that the initial protective dielectricmaterial 46 a is completely removed from horizontal surfaces (e.g.,surfaces formed in the x-y plane including the top surface of the lowerline stack 44 c and the substrate surface between adjacent lower linestacks 44 c) of exposed surfaces while the initial protective dielectricmaterial 46 a remains over vertical surfaces (e.g., surfaces formed inthe x-z and x-y planes). Spacer etch processes can include sputteretching or reactive ion etching, for example.

In the illustrated embodiment of FIG. 2D, removing at least an upperportion of the initial protective dielectric material includesperforming an extended spacer etch to additionally remove the initialprotective dielectric 46 a from an upper portion of the lower line stack44 b of FIG. 2C to expose an upper portion of sidewalls of the lowerline stack 44 b. The exposed upper portion of sidewalls includessidewalls of the storage element line 34 b. In the illustratedembodiment, exposed upper portion of sidewalls additionally includessidewalls of the first hard mask line 42 b if still present, sidewallsof the upper electrode line 32 b, and at least a portion of thesidewalls of the middle electrode line 36 b. On the other hand, a lowerportion of sidewalls of the lower line stack 44 b that remains incontact with and covered by the protective dielectric material 46 bincludes at least sidewalls of the selector element line 38 b. In theillustrated embodiment, the lower portion of the sidewalls of the lowerline stack 44 b that remain covered additionally includes at least aportion of sidewalls of the middle electrode line 36 b, sidewalls of thelower electrode line 40 and sidewalls of the lower conductive line 22.

Still referring FIG. 2D, according to some embodiments, the method offabricating the memory cell stack additionally includes cleaning theexposed portions of sidewalls of the lower line stack 44 c whilecontinuing to protect the sidewalls of the covered portions of sidewallsof the lower line stack 44 c. In the illustrated embodiment, the methodincludes cleaning all exposed surfaces of the first hard mask line 42 bif still present, sidewalls of the upper electrode line 32 b, sidewallsof the storage element line 34 b, and at least an upper portion of thesidewalls of the middle electrode line 36 b not covered by the initialprotective dielectric 46 b. The surfaces that remain covered, however,including at least a lower portion of the sidewalls of the middleelectrode line 36 b and sidewalls of all layers below the middleelectrode line 36 b, are protected from being exposed to a cleaningagent. Thus, as discussed above, while contaminants on and/or materialsof the storage element line 34 b may be removed from the sidewalls ofthe storage element line 34 b by the cleaning agent, the selectorelement line 38 b remains protected from being exposed to the cleaningagent. By way of an example, sidewalls of the storage element line 34 bthat may be contaminated by atoms, molecules, or clusters of theselector element material (e.g., Se) can be cleaned without removing torelease additional atoms, molecules, or clusters of the selector elementmaterial (e.g., Se).

In various embodiments, cleaning the exposed portions of sidewalls ofthe lower line stack 44 c can be performed using any suitable wet or drycleaning processes known in the art for cleaning dry-etched surfaces. Insome embodiments, the cleaning processes may include wet and/or vaporcleaning in a solution including carboxylic acid and/or dilutehydrofluoric acid (HF). For example, the solution may include dilutehydrofluoric acid (HF) at a concentration between about 0% and 10%, orbetween about 0% and 5%, and/or carboxylic acid at a concentrationbetween about 0% and about 2%, or between about 0% and about 1%, orbetween about 0% and about 0.5%. [0039] It will be appreciated thatdespite the particular example given above of protecting the selectorelement line 38 b while exposing the storage element line 34 b, themethod described herein is generally applicable to protect any coveredlower portion of the lower line stack 44 c from atoms, molecules, orclusters of materials from any exposed upper portion of the lower linestack 44 c. In other embodiments, for example, a lower memory storageelement may be protected while surfaces of an upper selector element arecleaned.

Referring to intermediate array structure 100 e of FIG. 2E, according tosome embodiments, after the initial protective dielectric material 46 a(FIG. 2C) is removed from at least an upper portion of the lower linestack 44 c to form the protective dielectric material 46 b (FIG. 2D),spaces between adjacent lower line stacks 44 c are filled with adielectric material to form first isolation dielectric regions 48 a. Inthe illustrated embodiment, the isolation dielectric regions 48 a are incontact the dielectric material 46 b and further in contact with upperportions of the lower line stack 44 c. Suitable dielectric materials tofill the spaces can include, for example, silicon oxide and siliconnitride, which may be deposited by suitable gap-filling processes suchas high-density plasma (HDP) processes, spin-on-dielectric (SOD)processes, sub-atmospheric chemical vapor deposition (SACVD) processes,and atomic layer deposition (ALD) processes, among others. Once theinter-line spaces between adjacent lower line stacks are filled with thedielectric material to form the isolation dielectric regions 48 a, theintermediate array structure 100 e can be chemical-mechanically polishedto expose a substantially planar surface comprising alternating surfacesof the upper electrode line 32 b interposed by isolation dielectricregions 48 a.

Referring to intermediate array structure 100 f of FIG. 2F, according tosome embodiments, the method of fabricating the memory cell stackadditionally includes depositing an upper conductive material 20 a. Theupper conductive material 20 a can comprise substantially similarmaterials and can be formed using substantially similar processes asdiscussed above for lower conductive material 22 a of FIG. 2A.

Referring to intermediate array structure 100 g of FIG. 2G, according tosome embodiments, the method of fabricating the memory cell additionallyincludes subtractively patterning to form an upper conductive line 20.Forming the upper conductive line 20, in a similar manner as describedabove for patterning the lower line stack 44 a (FIG. 2B), includesforming an etch mask pattern comprising lines (e.g. photoresist lines,not shown for clarity) extending in the y-direction and etching theexposed regions to form the upper conductive line 20. As with the lowerconductive lines 22, only short segments of two such upper conductivelines 20 are shown, spanning two memory cells each. The skilled artisanwill appreciate that, in practice many parallel lines may be formed tospan memory array.

Still referring to FIG. 2G, the method of fabricating the memory cellstack according to the illustrated embodiment additionally includesfurther subtractively etching between the mask line patterns extendingin the y-direction to form a wall structure 50 a. The wall structure 50a includes the upper conductive line 20 and a two-dimensionallylaterally confined stack including an upper electrode 32, a storageelement 34, a middle electrode 36 and a selector element 38 disposedunder the upper conductive line 20. The stack can also be referred to asa pillar and is referred to herein as a cell stack 30 c. The wallstructure 50 a further includes isolation dielectric regions 48 binterposed between adjacent cell stacks 30 c. In the illustratedembodiment of FIG. 2G, etching is stopped after etching to form theselector element 38 such that the resulting cell stack 30 c formedbetween the upper conductive line 20 and the lower conductive line 22includes the upper electrode 32, the storage element 34, the middleelectrode 36, the selector element 38, and the lower electrode line 40.The subtractive etching removes the first protective dielectric material46 b (Fig. FIG. 2F) between adjacent walls 50 a above the lowerelectrode line 40 to form the first protective dielectric material 46shown in FIG. 2G. As described above, however, the etching can bestopped after etching any layer of the wall structure such that any oneof the cell stack component layers can form a line similar to the lowerelectrode line 40.

Referring to intermediate array structure 100 h of FIG. 2H, according tosome embodiments, the method of fabricating the memory cell stackpartially covered with protective dielectric materials additionallyincludes forming a second initial protective dielectric material 52 aover the wall structure 50 a of FIG. 2G to form a covered wall structure50 b. The illustrated covered wall structure 50 b includes the wallstructure 50 a of FIG. 2H that is uniformly covered with the secondinitial protective dielectric material 52 a.

In some embodiments, similar to FIG. 2C, forming the second initialprotective dielectric material 52 a includes depositing a conformalprotective dielectric material which uniformly and substantially coversall exposed surfaces of the intermediate structure 100 g of FIG. 2G,including the sidewalls of the wall structure 50 a and inter-wallregions between adjacent wall structures 50 a. In addition, the secondinitial protective dielectric material 52 a can include similarmaterials and can be deposited using similar techniques as the initialprotective dielectric material 46 a discussed above in reference to FIG.2C.

Referring to intermediate array structure 100 i of FIG. 21, according tosome embodiments, the method of fabricating the memory cell stackadditionally includes removing at least an upper portion of the secondinitial protective dielectric material 52 a of FIG. 2H. Partial removalexposes at least an upper portion of the sidewalls of the wall structure50 a (FIG. 2G) to leave a partially covered wall structure 50 c, in asimilar manner to that described with respect to FIG. 2D. The exposedupper portion of the sidewalls of the partially covered wall structure50 c includes at least sidewalls of the storage element 34. In theillustrated embodiment, the exposed upper portion additionally includessidewalls of the upper conductive line 20, sidewalls of the upperelectrode 32, and at least a portion of sidewalls of the middleelectrode 36. The resulting second protective dielectric material 52remains in contact to cover at least a lower portion of the sidewalls ofthe wall structure 50 c. The lower portion of the sidewalls of thepartially covered wall structure 50 c that remains in contact with andcovered by the second protective dielectric material 52 includes atleast sidewalls of the selector element 38. In the illustratedembodiment, the covered portion additionally includes sidewalls of atleast a portion of the middle electrode 36.

In some embodiments, removing at least an upper portion of the secondinitial protective dielectric material 52 a to form the secondprotective dielectric material 52 includes performing a spacer etchprocess similar to that described above with respect to FIG. 2D. In theillustrated embodiment of FIG. 21, performing the spacer etch includesremoving the second initial protective dielectric 52 a from an upperportion of the covered wall structure 50 b (FIG. 2H) such that resultingpartially covered wall structure 50 c of FIG. 21 exposes sidewalls ofthe storage element 34 extending in the y-direction. In someembodiments, the spacer etch additionally exposes sidewalls of the upperconductive line 20 extending in the y-direction, sidewalls of the upperelectrode 32 extending in the y-direction, and at least a portion of thesidewalls of the middle electrode 36 extending in the y-direction. Theremaining second protective dielectric material 52 covers at least aportion of the sidewalls of the middle electrode 36 extending in they-direction and sidewall surfaces of layers below the middle electrodeline 36, including the selector element 38. In addition, due to thedirectional nature of the spacer etch process, the protective dielectricmaterial 52 a (FIG. 2H) is removed from an upper surface of the lowerelectrode line 40 to expose portions of the upper surface betweenpartially covered wall structures 50 c.

Still referring FIG. 21, according to some embodiments, fabricating thememory cell stack additionally includes cleaning the exposed portions ofsidewalls of the partially covered wall structure 50 c while continuingto protect the sidewalls of the selector element 38 covered by theprotective dielectric material 52 b. Similar to FIG. 2D, the methodincludes cleaning exposed surfaces of the upper conductive line 20,exposed sidewalls of the upper electrode 32, exposed sidewalls of thestorage element 34 and exposed portions of the sidewalls of the middleelectrode 36, as well as exposed upper surface of the lower electrodeline 40. The surfaces that remain covered, however, including at least aportion of the sidewalls of the middle electrode 36 extending they-direction and sidewall surfaces of the selector element 38 extendingin the y-direction, remain protected from the cleaning agent. Thus,similar to the cleaning process described with respect to FIG. 2D, whilecontaminants from the exposed sidewalls of the storage element 34 may beremoved by the cleaning agent, all sidewall surfaces of the selectorelement 38 remain protected from the cleaning agent. Also similar to thecleaning process described with respect to FIG. 2D, sidewalls of thestorage element 34 that may be contaminated by atoms, molecules, orclusters of the selector element material (e.g., Se) can be cleanedusing any suitable wet or dry cleaning processes described with respectto in FIG. 2D, without risking the release of additional atoms,molecules, or clusters of the selector element material (e.g., Se).

Still referring to FIG. 21, according to some embodiments, while notshown for clarity, after cleaning the exposed sidewalls of the partiallycovered wall structure 50 c, spaces between adjacent wall structures 50c can be filled with a suitable dielectric material in a similar mannerto that described in FIG. 2E. Once the spaces between adjacent partiallycovered wall structures 50 c are filled with the dielectric material,the intermediate array structure 100 i can be chemical-mechanicallypolished to expose a substantially planar surface comprising alternatingsurfaces of the upper conductive lines 20 interposed by isolationdielectric regions.

In the embodiment illustrated in FIGS. 2A-2I, the protective dielectricmaterial 46 is formed on first sidewalls extending in the x-direction ofthe intermediate array structure 100 b (FIG. 2B) and the secondprotective dielectric material 52 is formed on second sidewallsextending in in the y-direction of the intermediate array structure 100g (FIG. 2G). However, other embodiments are possible, where protectivedielectric material is formed on one of the first or second sidewalls.FIGS. 3A-3C illustrates these alternative embodiments, incross-sectional views.

FIGS. 3A-3C illustrate memory devices 200 a, 200 b, and 200 c eachcomprising a memory cell stack 30 formed on a substrate, according toembodiments. The memory cell stack 30 can be similar to the memory cellstack 30 of FIG. 1. For clarity, in FIGS. 3A-3C illustrate views of thememory cell stack 30 of cross-sections made in the x-y plane, cut acrossa lower portion of the memory cell stack covered by at least one of theprotective dielectric material 46 (FIG. 2D) and the second protectivedielectric material 52 (FIG. 21). That is, FIGS. 3A-3C can representcross sections made in the x-y plane across the selector element 38 inFIG. 1 with at least one of the protective dielectric material 46 (FIG.2D) and the second protective dielectric material 52 (FIG. 21) formed onone or both of the sidewalls. Similar to FIG. 1, each of the cell stacks30 in FIGS. 3A-3C comprises a first memory element, which can be aselector element 38, and a second memory element formed over the secondmemory element, which can be a storage element 34. Similar to FIG. 1,the first memory element and the second memory element can be separatedby a middle electrode 36. The cell stack 30 can additionally include afirst electrode 40 between the selector element 38 and the row line 22,and can further include a second electrode 32 between the column line 20and the storage element 34.

FIG. 3A illustrated the memory device 200 a including a plurality ofmemory cell stacks 30, according to an embodiment. Each of the memorycell stacks 30 comprises a first pair of sidewalls 60 a extending in thex-direction and opposing each other, and a second pair of sidewalls 60 bextending in the y-direction and opposing each other. The memory device200 a further includes protective dielectric insulating materials 46formed on at least a portion of the first pair of sidewalls 60 a. Theprotective dielectric insulating materials 46 can be formed on a lowerportion of the first pair of sidewalls 60 a such that the protectivedielectric insulating materials 46 are formed on the first memoryelement. In addition, the protective dielectric insulating materials 46can be formed on sidewalls of the first memory element while not formedon sidewalls of the second memory element. In addition, the protectivedielectric insulating materials 46 can extend parallel to the row line22 in the x-direction and coterminate with the first memory element inthe x-direction at the second pair of sidewalls 60 b. In addition, thememory device 200 a can further include an isolation dielectric 48formed on the protective dielectric insulating materials 46 and on anupper portion of the first pair of sidewalls 60 a, and further formed onthe second pair of sidewalls 60 b, such that the isolation dielectricmaterial 48 laterally surrounds each of the memory cell stacks 30.

FIG. 3B illustrates the memory device 200 b including a plurality ofmemory cell stacks 30, according to another embodiment. Similar to FIG.3A, each of the memory cell stacks 30 comprises a first pair ofsidewalls 60 a extending in the x-direction and opposing each other, anda second pair of sidewalls 60 b extending in the y-direction andopposing each other. Unlike FIG. 3A, the memory device 200 b furtherincludes protective dielectric insulating materials 52 formed on atleast a portion of the second pair of sidewalls 60 b. The protectivedielectric insulating materials 52 can be formed on a lower portion ofthe second pair of sidewalls 60 b such that the protective dielectricinsulating materials 52 are formed on the first memory element. Inaddition, the protective dielectric insulating materials 52 can beformed on sidewalls of the first memory element while not formed onsidewalls of the second memory element. In addition, the protectivedielectric insulating materials 52 can extend parallel to the columnline 20 in the y-direction. Unlike FIG. 3A, the protective dielectricinsulating materials 52 do not coterminate with the first memory elementin the y-direction at the first pair of sidewalls 60 a. In addition, thememory device 200 b can further include an isolation dielectric 48formed on the protective dielectric insulating materials 52 and on anupper portion of the second pair of sidewalls 60 b, and further formedon the first pair of sidewalls 60 a such that the isolation dielectricmaterial 48 laterally surrounds each of the memory cell stacks 30.

FIG. 3C illustrated the memory device 200 c including a plurality ofmemory cell stacks 30, according to yet another embodiment. Similar toFIGS. 3A and 3B, each of the memory cell stacks 30 comprises a firstpair of sidewalls 60 a extending in the x-direction and opposing eachother, and a second pair of sidewalls 60 b extending in the y-directionand opposing each other. In FIG. 3C, the memory device 200 c furtherincludes first protective dielectric insulating materials 46 formed onat least a portion of the first pair of sidewalls 60 a, similar to FIG.3A, and further includes second protective dielectric insulatingmaterials 52 formed on at least a portion of the second pair ofsidewalls 60 b, similar to FIG. 3B. That is, in FIG. 3C, the memory cellstacks 30 are laterally surrounded by dielectric insulating materials 46and 52. Also similar to FIGS. 3A and 3B, the first and second protectivedielectric insulating materials 46 and 52 can be formed on lowerportions of the first and second pairs of sidewalls 60 a and 60 b,respectively, such that the protective dielectric insulating materials46 and 52 surround the first memory element. In addition, the first andsecond protective dielectric insulating materials 46 and 52 can beformed on the first and second sidewalls of the first memory elementwhile not formed on the first and second sidewalls of the second memoryelement. In addition, similar to FIG. 3A, the first protectivedielectric insulating materials 46 can extend parallel to the row line22 in the x-direction and coterminate with the first memory element inthe x-direction at the second pair of sidewalls 60 b, while similar toFIG. 3B, the second protective dielectric insulating materials 52 canextend parallel to the column line 20 in the y-direction and do notcoterminate with the first memory element terminating in the y-directionat the first pair of sidewalls 60 a. In addition, the memory device 200c can further include an isolation dielectric 48 formed on theprotective dielectric insulating materials 46 and on an upper portion ofthe first pair of sidewalls 60 a, and further formed on the protectivedielectric insulating materials 52 and on an upper portion of the secondpair of sidewalls 60 b.

Although this invention has been described in terms of certainembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Moreover, the various embodiments described above can becombined to provide further embodiments. In addition, certain featuresshown in the context of one embodiment can be incorporated into otherembodiments as well. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

1. (canceled)
 2. A apparatus, comprising: a substrate; a memory cellstack over the substrate, the memory cell stack comprising: a firstprotective material formed on at least a portion of a first pair ofsidewalls of the memory cell stack; and a second protective materialformed on at least a portion of a second pair of sidewalls of the memorycell stack, the second protective material intersecting the firstprotective material.
 3. The apparatus of claim 2, wherein the memorycell stack further comprises a first memory element and a second memoryelement, and wherein the first protective material and the secondprotective material are formed on at least a portion of the first memoryelement.
 4. The apparatus of claim 4, wherein the first protectivematerial and the second protective material are formed on a portion ofthe memory cell stack, and the second memory element is above theportion of the memory cell stack.
 5. The apparatus of claim 2, whereinthe first protective material and the second protective materialcomprise a same dielectric insulating material.
 6. The apparatus ofclaim 2, wherein the first protective material comprises a firstdielectric insulating material and the second protective materialcomprises a second dielectric insulating material that is different thanthe first dielectric insulating material.
 7. The apparatus of claim 2,wherein the first protective material comprises at least one of SiO₂,Al₂O₃ and Si₃N₄ and the second protective material comprises at leastone of SiO₂, Al2O₃ and Si₃N₄.
 8. The apparatus of claim 2, wherein thefirst protective material comprises SiO₂ and the second protectivematerial comprises at least one of Al₂O₃ and Si₃N₄.
 9. The apparatus ofclaim 2, wherein the first protective material comprises Al₂O₃ and thesecond protective material comprises at least one of SiO₂ and Si₃N₄. 10.The apparatus of claim 2, wherein the first protective materialcomprises Si₃N₄ and the second protective material comprises at leastone of SiO₂ and Al₂O₃.
 11. A apparatus, comprising: a first memoryelement of a memory cell stack; a second memory element over the firstmemory element; a first protective material formed on at least a firstportion of the first memory element, the first protective materialextending in a first direction; and a second protective material formedon at least a second portion of the first memory element, the secondprotective material extending in a second direction crossing the firstdirection.
 12. The apparatus of claim 11, wherein the second directionis perpendicular to the first direction.
 13. The apparatus of claim 11,wherein the first protective material comprises a first dielectricinsulating material and the second protective material comprises asecond dielectric insulating material the same as the first dielectricinsulating material.
 14. The apparatus of claim 11, wherein the firstprotective material comprises a first dielectric insulating material andthe second protective material comprises a second dielectric insulatingmaterial different than the first dielectric insulating material. 15.The apparatus of claim 11, wherein the first protective material and thesecond protective material each comprise at least one of SiO₂, Al₂O₃ andSi₃N₄.
 16. A method, comprising: forming a first protective material onat least a portion of a first pair of sidewalls of a memory cell stack,the first protective material extending in a first direction; andforming a second protective material on at least a portion of a secondpair of sidewalls of the memory cell stack, the second protectivematerial extending in a second direction different than the firstdirection associated with the first protective material.
 17. The methodof claim 16, wherein the second direction is orthogonal to the firstdirection.
 18. The method of claim 16, wherein forming the firstprotective material further comprises: applying chemical vapordeposition (CVD), atomic layer deposition (ALD), a thermal depositiontechnique, a plasma deposition technique, or a combination thereof. 19.The method of claim 16, wherein the first protective material and thesecond protective material comprise a same dielectric insulatingmaterial.
 20. The method of claim 16, wherein the first protectivematerial and the second protective material comprise differentdielectric insulating materials.
 21. The method of claim 16, wherein thefirst protective material and the second protective material eachcomprise at least one of SiO₂, Al₂O₃ and Si₃N₄.